The invention relates to an apparatus for evaporating liquids which comprises at least one heatable wall and means for distributing the liquid concerned over the said wall in the form of a thin film. Such apparatus may be used in industry, for example, in systems in which for the transfer of heat, a liquid is evaporated at one location (evaporator) and then is condensed at another location (condenser). Owing to the good heat transfer during evaporation and condensation, the heat transfer areas required are small. Furthermore the temperature losses and the circulating amount of the heat transfer medium are small. The latter facts have the advantage that circulating the medium requires only little power.
Since the temperatures at which the processes occur in chemical processing technology and also in engines generally are very high, the temperature level at which the aforementioned heat transfer is effected must be high also. At these high temperatures of more than 500.degree.C, metallic heat carriers (for example Na and K) are suitable, because they are capable of withstanding these temperatures and have low vapor pressures. However, a disadvantage is that boiling of these liquids is retarded by their high surface tension and the small increase of their vapor pressure with temperature. This gives rise to undesirable fluctuations of the temperature and the pressure. To avoid these disadvantages, it is known to convey such liquids in the form of a thin film over a heated surface so that evaporation at the film surface takes place substantially without overheating.
At the very high temperatures the distribution of the liquid over the heated surface gives rise to difficulty. At low temperatures the thickness of the layer of liquid is usually reduced by wipers or rotating rollers. For technological reasons, it is difficult to use these methods at elevated temperatures. Furthermore down-flow (fall-stream) evaporators are known in which the liquid is distributed over a wall from the top thereof to flow down the wall. However, it is very difficult to obtain undisturbed layers of even thickness.
It is an object of the present invention to obviate the aforementioned problems.